Configuring flow-control

For details on how flow-control works, see Managing incoming and outgoing messages with flow-control. The summary of the main points is as follows:

• The AxoSyslog application normally reads a maximum of log-fetch-limit() number of messages from a source.

• From TCP and unix-stream sources, AxoSyslog reads a maximum of log-fetch-limit() from every connection of the source. The number of connections to the source is set using the max-connections() parameter.

• Every destination has an output buffer. The size of this buffer is set automatically for log paths that use flow-control, and can be set using the log-fifo-size() option for other log paths.

• Flow-control uses a control window to determine if there is free space in the output buffer for new messages. Every source has its own control window, the log-iw-size() option sets the size of the static control window. Optionally, you can enable a dynamic control window for the source using the dynamic-window-size() option.

• When a source accepts multiple connections, the size of the control window is divided by the value of the max-connections() parameter and this smaller control window is applied to each connection of the source.

  The dynamic control window is automatically distributed among the active connections of the source.

• If the control window is full, AxoSyslog stops reading messages from the source until some messages are successfully sent to the destination.

• If the output buffer becomes full, and neither disk-buffering nor flow-control is used, messages may be lost.

Example: Sizing parameters for flow-control

Suppose that AxoSyslog has a source that must accept up to 300 parallel connections. Such situation can arise when a network source receives connections from many clients, or if many applications log to the same socket.

Set the max-connections() parameter of the source to 300. However, the log-fetch-limit() (default value: 10) parameter applies to every connection of the source individually, while the log-iw-size() (default value: 1000) parameter applies to the source. In a worst-case scenario, the destination does not accept any messages, while all 300 connections send at least log-fetch-limit() number of messages to the source during every poll loop. Therefore, the control window must accommodate at least max-connections()*log-fetch-limit() messages to be able to read every incoming message of a poll loop. In the current example this means that log-iw-size() should be greater than 300*10=3000. If the control window is smaller than this value, the control window might fill up with messages from the first connections — causing AxoSyslog to read only one message of the last connections in every poll loop.

The output buffer of the destination must accommodate at least log-iw-size() messages, but use a greater value: in the current example 3000*10=30000 messages. That way all incoming messages of ten poll loops fit in the output buffer. If the output buffer is full, AxoSyslog does not read any messages from the source until some messages are successfully sent to the destination.

   source s_localhost {
        network(
            ip(127.0.0.1)
            port(1999)
            max-connections(300)
        );
    };
    destination d_tcp {
        network("10.1.2.3"
            port(1999)
            localport(999)
            log-fifo-size(30000)
        );
    };
    log {
        source(s_localhost);
        destination(d_tcp);
        flags(flow-control);
    };

If other sources send messages to this destination, then the output buffer must be further increased. For example, if a network host with maximum 100 connections also logs into the destination, then increase the log-fifo-size() by 10000.

   source s_localhost {
        network(
            ip(127.0.0.1)
            port(1999)
            max-connections(300)
        );
    };
    source s_tcp {
        network(
            ip(192.168.1.5)
            port(1999)
            max-connections(100)
        );
    };
    destination d_tcp {
        network("10.1.2.3"
            port(1999)
            localport(999)
            log-fifo-size(40000)
        );
    };
    log {
        source(s_localhost);
        destination(d_tcp);
        flags(flow-control);
    };